1. Field of the Invention
The present invention relates to preventing or mitigating scale formation during oil and gas production. The present invention particularly relates to preventing or mitigating the formation of sulfide scales during oil and gas production.
2. Description of the Prior Art
The forming of scale during the production and transportation of crude oil has long been a problem. The formation of scale can slow oil production rate and, in extreme circumstances, stop production completely. Scale may also be a major problem in downstream equipment such as separators, desalters, pipelines and the like. Even pipelines and, in offshore fields, flow lines may be adversely affected by scale.
Some scale species are formed from the interaction of metals and inorganic anions. For example, the combination products of calcium cations with carbonate anions or phosphate anions will precipitate from the water in which the ions are carried to form scale deposits when the concentrations of these anions and cations exceed the solubility of the reaction product.
Other common scale forming ions are calcium and barium, but sodium, carbonate, bicarbonate, chloride, sulfate, and strontium are also recognized as scaling species. The most common speciation of these combined scaling ions are: calcium carbonate (CaCO3), calcium sulfate (CaSO4), barium sulfate (BaSO4), and strontium sulfate (SrSO4).
In addition to the scaling species described above, crude oil will often include other materials which may form scale or scale like deposits down stream. For example, most crude oil will have agglomerative materials such as paraffins and asphaltenes. Most crude oil will also have clays and silica. All of these materials are known to contribute to problems with downstream equipment used to move and process crude oil
Sulfide scales, especially iron sulfide (FeS) can cause significant production problems and safety hazards for producers. Iron sulfide scale often causes restricted production by plugging flow paths in the reservoir, perforations, pump intakes, and tubulars. This problem may be especially acute in reservoirs flooded with water containing significant sulfates. The influx of sulfate can stimulate indigenous sulfate reducing bacteria, which metabolize the sulfate into hydrogen sulfide gas. The hydrogen sulfide then reacts with metallic compounds such as iron to form iron sulfide, which appears as a black scale. Producers often use hydrochloric acid treatments to clean up iron sulfide scale. These treatments can produce deadly H2S gas and are usually expensive. If water disposal systems are required to handle sour produced water, the maintenance costs will increase even further as the pumps, filtration systems, injection lines, and injection wells are attacked by sulfide scale and corrosion.